The invention relates to a direction sensitive flow velocity meter for a fluid, either a gas or a liquid, comprising a sensing plate with a sensor part and with devices for generating heat, and electronic means for converting the electric signals furnished by the sensor part. The invention likewise relates to a sensing plate to be used in such a flow velocity meter.
A similar flow velocity meter is known from Dutch patent application No. 7609696. The known velocity meter contains as sensing plate a chip on which the sensor part and a signal converting part are integrated. The sensor part in this device consists of two or more temperature sensitive transistors, of which, seen in the flow direction, at least one is situated upstream and at least one is situated downstream on the chip. When the chip is heated, the flowing fluid, which first meets the upstream transistor or transistors on the chip, will be heated when flowing along the chip and will in turn cause the downstream transistor or transistors on the chip to attain a higher temperature than the upstream transistor or transistors. This causes a difference in output signal between the upstream and the downstream transistors. This difference in output signal is an indication of the flow velocity.
Although satisfactory results are achieved with the known meter, there are some inherent problems. The measuring transistors exhibit, as do all transistors, a drift phenomenon, that is to say, that with time the output signal produced by the temperature sensitive transistors at a certain temperature difference between the upstream and downstream edges of the chip can vary. It is true that the phenomenon is such that only very slight temperature differences, in the order of hundredths of degrees seem to be at issue, but at flow velocity measurements in slowly moving fluids such a deviation may already be in the order of magnitude of the measurement itself. Obviously it is possible to periodically re-calibrate the meter, at least to compensate for the drift phenomena, but in a number of cases, for instance in the chemical process industry, it is undesirable once it has been installed, to dismantle a meter for some time to re-calibrate it.
One of the the objects of this invention is to offer a solution for the problem indicated by providing a flow velocity meter in which there are no time-related drift phenomena in the sensor part or in which such drift phenomena have no noticeable effect.